1. Technical Field
This invention relates to the transmission of information employing a burst of multiple frequency signals such as for example in orthogonal frequency division multiplexing (OFDM). It particularly though not necessarily exclusively relates to the performance of OFDM by a modem intended for subsea communication, such as by means of an umbilical including a transmission line, which may be of significant length.
2. Related Art
Although transmissions of this kind, particularly OFDM, have become popular in a variety of contexts, particularly wireless networks, a problem exists for the reliable acquisition and decoding of the transmitted signal. In particular, in view of the broadband structure of the burst, it is difficult to determine precisely the start of a burst.
If a start signal can be reliably determined to provide an accurate datum, then the burst may commence a predetermined time interval after the datum so as to allow appropriate synchronization of the receiver. A significant problem, particularly for subsea communications, is posed by the impulse response of the transmission line and the dispersion of the components of a start signal. For example, a start signal composed of a short pulse is simply represented in the time domain but in the frequency domain has a substantial spectrum which can be come seriously distorted by transmission along an umbilical of substantial length. The problem is accentuated if the transmission line is constituted by power lines on which the relatively high frequency signals are imposed in addition to the electrical power signals carried by the power lines.
The state of the art in relation to synchronization of OFDM systems (particularly in relation to wireless broadcast systems) is represented by US-6993084-B1, WO99/5366-A1 and IEEE 47th Vehicular Technology Conference, May 1997 pages 2094-2098 (Hazy et al), ‘Synchronization of OFDM systems over frequency selective fading channels’.